The most common and most likely answer to this crossword clue is the 6 plus 5 letter word LONELY HEART
I've seen this crossword clue in The Daily Telegraph
L
O
N
E
L
Y
H
E
A
R
T
More clues leading to the result LONELY HEART
Buddy explains!
The straight part
We know that "One left? That might summarise this unhappy singleton" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one left? that might summarise this unhappy singleton = lonely heart.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.